Originally Posted by
Chaltione
According to the mount achievement pages on wowhead, your current achievement status is based on the highest on all of your characters, but your current character might lack some class mounts.
Example, you see 199/200 on your non-dk/paladin/warlock, but only have 197 mounts available to that character. You learn one more, and have 198 available on that character. Once you log on to a dk/paladin/warlock with the class mounts, you'll get the achievement as they will have 200.